The general area of our reserach interests is the regulation of the contractile activity of smooth muscle. In particular we are concerned with those regulatory processes which influence directly the enzymatic properties of actomyosin. The regulatory mechanism in smooth muscle is quite distinct from that found in skeletal muscle, and instead of being linked to the actin filament is thought to be directed towards the myosin molecule. The most popular theory is based on the reversible phosphorylation of two of the myosin light chains. We intend to evaluate the phosphorylation theory, and to ask if phosphorylation alone can account for all of the observed enzymatic properties of actomyosin. Preliminary evidence indicates that additional complimentary factors are involved and our other main objective is to identify these factors and ultimately to establish their mechanism of action. It is possible that the complimentary system is leiotonin. One of the major controversies in smooth muscle biochemistry is whether regulation occurs via the phosphorylation mechanism or via the leiotonin system, and it is hope that our results will help to resolve this argument.